1. Field of the Invention
The present invention relates to a catalyst for combustion of inflammable gases, and it relates to, in particular, a poisoning-resistant catalyst used for purifying a desired gas by removing inflammable impurities (e.g. CO, H.sub.2, hydrocarbons, etc.) in a starting gas by a catalytic combustion method, and a process for producing this catalyst.
2. Description of Related Art
For production of semiconductor devices, various gases are used. In recent years, there has been a growing demand for an ultrahigh-purity gas with an enhancement of the degrees of integration and fineness of semiconductor devices, for the purpose of improving the yield of a product. In particular, nitrogen gas used as a carrier gas or a gas for conditioning an atmosphere has also come to be increasingly required to have an ultrahigh purity, and the permissible content of impurities in the nitrogen gas decreases at an increasing tempo.
In general, high-purity nitrogen gas is collected by separating nitrogen gas from air by subzero fractionation, but a slight amount of CO contained in the starting air cannot be removed by the subzero fractionation because its boiling point is near that of nitrogen. Therefore, CO is burned by the use of a catalyst before the subzero fractionation, and then separated and removed.
Processes for purifying a desired gas by removing such an inflammable gas in a starting gas by a catalytic combustion method have been disclosed, for example, in Japanese Patent Application Kokoku No. 3-7603 and Kokai No, 61-225568 and Japanese Utility Model Kokai No. 61-123389. They comprise converting CO or H.sub.2, i.e., an inflammable gas contained in a starting gas, into CO.sub.2 or H.sub.2 O, respectively, by catalytic combustion, separating the CO.sub.2 or H.sub.2 O by means of an adsorption separator provided in the downstream side of the catalytic combustor, eliminating the same from the system, separating the starting air freed of CO or H.sub.2, by a subzero fractionation apparatus, and thereby producing a desired high-purity gas.
As a catalyst used in the above catalytic combustion method, there are generally used noble metal catalysts such as Pt and Pd which have a high activity in a low temperature range. However, when a component poisonous to the catalyst (e.g. SO.sub.2 or H.sub.2 S) is contained in the starting gas, the catalyst is poisoned to be decreased in activity. The decrease of the activity results in a lowered removal rate of the inflammable gas in the starting gas, so that the purity of the gas produced by means of the subzero fractionation apparatus is lowered.
As a means for solving this problem, there is a method in which poisoning of the catalyst is prevented by providing a means for removing the poisonous component, on the inlet side of a catalyst tank. For example, a method for removing SO.sub.2 gas, a main component poisonous to the catalyst, has been proposed (Japanese Patent Application Kokai No. 2-119921). However, the provision of a means for removing such a component poisonous to the catalyst increases the cost of production of a catalytic combustion apparatus.
As a method for preventing the catalyst from being poisoned, employment of a high reaction temperature in a catalyst tank is known. In detail, as compared with catalytic combustion at a low temperature, catalytic combustion at a high temperature makes difficult the adsorption of a component poisonous to the catalyst and hence permits suppression of the decrease of activity of the catalyst. However, the employment of the high reaction temperature requires preheating of the starting gas or the catalyst as much, resulting in an increased running cost.